Constantine I (Greek: Κωνσταντίνος Αʹ, Konstantínos I; 2 August [O.S. 21 July] 1868 – 11 January 1923) was King of Greece from 18 March 1913 to 11 June 1917 and from 19 December 1920 to 27 September 1922. He was commander-in-chief of the Hellenic Army during the unsuccessful Greco-Turkish War of 1897 and led the Greek forces during the successful Balkan Wars of 1912–1913, in which Greece expanded to include Thessaloniki, doubling in area and population. He succeeded to the throne of Greece on 18 March 1913, following his father's assassination.

Constantine’s disagreement with Eleftherios Venizelos over whether Greece should enter World War I led to the National Schism. He forced Venizelos to resign twice, but in 1917 he left Greece, after threats by the Entente forces to bombard Athens; his second son, Alexander, became king. After Alexander's death, Venizelos' defeat in the 1920 legislative elections, and a plebiscite in favor of his return, Constantine was reinstated. He abdicated the throne for the second and last time in 1922, when Greece lost the Greco-Turkish War of 1919–1922, and this time was succeeded by his eldest son, George II. He died in exile four months later, in Sicily.